Swainson's Hawks in California

The Swainson's hawk (Buteo Swainsoni) was listed as a threatened species in 1983 by the California Fish and Game Commission. This listing was based on loss of habitat and decreased numbers across the state.

Distribution and Abundance

It is thought that the historic population of Swainson's hawks in California was as many as17,136 pairs. In 1980 a report developed by Bloom (PDF) estimated 375 (+50) breeding pairs of Swainson's hawks remaining in California. Bloom's report noted number to the greatest in the Central Valley and in the Great Basin area of northeastern California, with a few Swainson's hawk territories located in Shasta Valley, the Owens Valley, and the Mohave Desert. In 1988 a Department led survey effort revealed no change in Swainson's hawk distribution from the 1980. The 1988 effort led to an estimate of 430 pairs in the Central Valley and a state-wide estimate of 550 breeding pairs. In 2005 a state-wide survey was conducted in the known range. The results showed a state-wide estimate for the number breeding pairs at 2081. Surveys conducted in Butte to San Joaquin counties during the period 2002-2009 showed numbers of breeding pairs of Swainson's hawks at 593 in 2002, 1008 in 2003 and 941 in 2009.

Biology

The Swainson's hawk is a medium-sized buteo with relatively long, pointed wings which curve up somewhat in a slight dihedral while the bird is in flight. The most distinctive identifying feature of adults is dark head and breast band distinctive from the lighter colored belly, and the underside of the wing with the linings lighter than the dark gray flight feathers. Adult females weigh between 900 and 1100 grams (32 to 39 oz), and males from 800 to 1000 grams (28 to 35 oz).

The Swainson's Hawk breeds in the western United States and Canada and winters in South America as far south as Argentina. A raptor adapted to the open grasslands, it has become increasingly dependent on agriculture, especially alfalfa crops, as native communities are converted to agricultural lands. The diet of the Swainson's hawk in California is varied, but mainly consists of small rodents called voles; however other small mammals, birds, and insects are also taken.

Swainson's Hawks often nest peripheral to riparian systems. They will also use lone trees in agricultural fields or pastures and roadside trees when available and adjacent to suitable foraging habitat. Swainson's Hawks in the Great Basin occupy the Juniper/Sagebrush community typical to the area.

Threats

The most recognized threat to Swainson's hawks in the loss of their native foraging and breeding grounds. As important foraging areas are converted to urban landscapes or other unsuitable habitat, the aptitude for the landscape to support breeding pairs decreases. Other threats include climate change, infrastructure placement, disease, pesticide poisoning, and electrocution.

Population Status and Trend

The surveys previously mentioned do not provide enough data to form a trend line. The Breeding Bird Survey (BBS) has long-term dataset that can be used to assess general population trends. According to the BBS Swainson's hawk data for California shows that between 1979 to the 2005 there may have been an increase in the population. This trend alone should not be solely relied upon for accurate trend data. Factors that may affect BBS trend data include an increased interest in the Swainson's hawk

Even while considering increase population estimates since the species was listed, the amount of increase is still far below the original population estimate of just over 17,000 Swainson's hawks statewide.